1. Field of the Invention
The present invention relates to a continuous process for making metal salt solutions which are particularly useful for agriculture or industry as fertilizers and industrial chemicals.
2. Description of the Prior Art
The current commercial method for making metal salt solutions is a batch process which is carried out by charging, e.g., an aqueous metal oxide/hydroxide slurry to a mixing tank equipped with an agitator and cooling means. A mineral acid, typically nitric acid, is added, slowly at first, then more rapidly toward the end of the reaction. This process is subject to a number of problems. The reaction can be violent and, on a large scale, even with good agitation and cooling, the reaction can "run away" and, in fact, at least one fatality has resulted recently from such a "run away" reaction. During the early addition of nitric acid, even if the reaction does not "run away", localized excessive heat release is often such that some nitric acid is decomposed and brown nitric oxide fumes are emitted from the reactor which can create a hazard if not properly absorbed and disposed of.
Another problem involved in the current process is that due to the slow addition of nitric acid, there is insufficient acid present in the initial part of the reaction to completely react with the metal oxide and the oxide particles become coated with nitrate salts. As a consequence, the current process provides very poor conversion of oxide to nitrate, yielding a substantial amount of sludge which must be disposed of. Since the sludge consists principally of metal oxide once nitric acid addition has ceased, there is a continued slow, variable reaction of the oxide which makes it difficult to produce a product with constant pH as is most preferred in the art.
Another problem which can also occur is over-acidulation and care must be taken in measuring the nitric acid to keep the reaction within specified limits.
U.S. Pat. No. 3,585,020 Legal, Jr. et al discloses a process for forming a free-flowing, granular, non-burning and non-crumbling 7-40-6 fertilizer composition. Reference is made at column 3 to the use of an in-line mixer. However, the use of spargers in this process suggest that it is specific to batch processing. In any case, Legal, Jr. et al is specific to forming granular materials quite different from the liquid solutions formed per the present invention.
U.S. Pat. No. 3,957,947 Yamada et al discloses a process for the continuous production of aqueous basic aluminum salt solutions. The products of Yamada appear to be deodorants, and while a short tubular reactor is involved, it is necessary to provide heat on an indirect basis and the overall reaction scheme is quite different from that of the present invention.